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Using a QEV with the chamber and the supply as one ?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Using a QEV with the chamber and the supply as one ?

Unread postAuthor: evilvet » Sun Jan 20, 2013 11:31 pm

I am now the proud owner of a basic eBay QEV, furthering my knowledge of pneumatic launchers for high speed photography here in wonderful land of Oz.

This is probably one for Poland or Wyz to shoot me down , so to speak.

Assume "P" supply side is connected via a slide valve to a shop compressor at 10 bar, about 50 litres. The "A" chamber is currently 150mm of 50mm steel pipe. "R" is feeding a 0.177" brass tube 300mm long.

Leaving aside issues such as C:B ratio etc (I used what I had available in the shed), what would be the result if any if I ran "A" back to to the shop compressor tank ?, using it as both supply and chamber.

Can this work, is it some sort of pneumatic Mobius strip, an Escher drawing of air or am I feeling the effects of a warm afternoon consuming Heineken ?

Cheers
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Re: Using a QEV with the chamber and the supply as one ?

Unread postAuthor: jackssmirkingrevenge » Mon Jan 21, 2013 1:22 am

evilvet wrote:Leaving aside issues such as C:B ratio etc (I used what I had available in the shed), what would be the result if any if I ran "A" back to to the shop compressor tank ?, using it as both supply and chamber.


Do you mean feeding through A and not through P?

In that case, it will never seal, because the piston is also a cup seal which will not allow air back into the pilot area.

If you feed through both A and P, it will fire once but never reseal.

In order for it to reseal, you have to replace or modify the piston, essentually poking a small hole in its rim in order to make sure it's not completely airtight, and a small light spring behind it also helps with resealing.
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Unread postAuthor: wyz2285 » Mon Jan 21, 2013 1:20 pm

Jack can you find a link to that thread where BTB and somebody talked about the alternative way of using a 3-way/slide valve as a pilot by blocking one of.the port? I can't find it. In that way he may not need to poke a hole if the chamber is big enough and the flow it's just right to suck the piston back( it can happen, I did it once). However, a little hole will make sure it work flawlessly.
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Unread postAuthor: POLAND_SPUD » Mon Jan 21, 2013 2:51 pm

jack I think he meants attaching the A to the compressor as well as having it connected to port P... at least that's my understanding

What will happen ? I think it will work as the piston area is larger at the pilot side so if pressure in on both sides is the same the piston will seal


evilvet care to explain why you want to set it up like that ?? it's not that you gain anything
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 21, 2013 3:06 pm

If the compressor is attached to a slide valve pilot and to the chamber, then shooting will continue to empty the compressor tank until the slide valve is reclosed...

evilvet care to explain why you want to set it up like that ?


Yup, that would help.
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Unread postAuthor: wyz2285 » Mon Jan 21, 2013 3:11 pm

Or connect directly the tank o the valve, mounting the slide valve backwards and block the end of the slide valve. Got my idea?
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Unread postAuthor: evilvet » Mon Jan 21, 2013 3:20 pm

Poland, JSR et al.

I think he meants attaching the A to the compressor as well as having it connected to port P


The idea popped in while I was searching for something to use as a temporary chamber for testing. I looked at the compressor and saw the two outlets, one regulated and one raw. It occurred to me that I could use the supply reservoir tank as the chamber as well. Obviously as Jack says it will continue to vent A-R until either the valve is reset or the tank empties.

I appeals from a neatness point of view, only one chamber/cylinder. Of course at some point the overall supply to the system needs to be refreshed via a separate fill port to the tank.
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 21, 2013 3:22 pm

wyz2285 wrote:Or connect directly the tank o the valve, mounting the slide valve backwards and block the end of the slide valve. Got my idea?


I don't remember the thread you had mentioned :?

If you mount the slide valve upside down and block the end...

- slide valve with ring moved away from the pilot, air can flow to the blocked end.

- side valve with ring moved towards the pilot, blocked end empties and nothing else happens.

That's what I understood, diagram maybe?
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Unread postAuthor: wyz2285 » Mon Jan 21, 2013 3:26 pm

By filling up the blocked end, short pressure drop trigger a small burst. Putting it back to position, exhaust the air trapped in the blocked end, preparing for another shot. I'm on my phone, can't make a diagram.
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 21, 2013 3:55 pm

Ahhhhh ok got it.

By trapping the pilot volume it would be great for a silent system ;)

You would still need to mod the piston for chamber filling though.
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Unread postAuthor: wyz2285 » Mon Jan 21, 2013 4:11 pm

The muzzle blast it's probably much more noticeable :lol:
The piston should be mod, but these rectangular QEV's diagram doesn't provide 100% airtight seal, if the pilot closes fast enough it won't dump all the air.
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Last edited by wyz2285 on Sun Feb 17, 2013 1:32 pm, edited 1 time in total.
CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 21, 2013 4:36 pm

wyz2285 wrote:The muzzle blast it's probably much more noticeable :lol:


No point in having a great silencer if you're dumping HPA out of the pilot though, which is why QDV/valveless systems are so attractive...

The piston should be mod, but these rectangular QEV's diagram does provide 100% airtight seal, if the pilot closes fast enough it won't dump all the air.


... but the air in the pilot will be at lower pressure, so as the air in the chamber increases it will act as a pop-off valve and either fizzle air out or go full auto ;)
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Unread postAuthor: wyz2285 » Mon Jan 21, 2013 4:48 pm

First, change the word does to doesn't, my mistake.
When the pilot side is full, the diagram should remain shut. But let's just say put a hole in there to end this :lol:
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Hotwired » Mon Jan 21, 2013 6:49 pm

wyz2285 wrote:By filling up the blocked end, short pressure drop trigger a small burst. Putting it back to position, exhaust the air trapped in the blocked end, preparing for another shot. I'm on my phone, can't make a diagram.


http://www.spudfiles.com/forums/viewtop ... tml#110053
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 22, 2013 1:52 am

Yup, pretty much what Hotwired had proposed... it should work well but in practice I don't see the advantage compared a conventional 3 way valve setup, aside from the fact that you don't need a separate firing chamber.
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